The invention relates to an electrode coating for a lithium ion battery, which contains a copolymer of vinyl-functional cyclic carbonate and acrylic acid or acrylic acid derivatives.
Owing to their high energy densities, lithium ion batteries are among the most promising energy stores for mobile applications. The fields of use extend from high-quality electronic appliances through to batteries for motor vehicles having electric drive and a stationary power store.
The development of high-performance anode materials for Li ion batteries requires the development of compatible binder systems at the same time. The PVDF used for graphite electrodes is not suitable for use silicon-containing electrodes because of chemical and mechanical instability. This shows up in a poor electrochemical cyclic behavior. To be able to accommodate the extreme volume change (up to about 300%), experienced by silicon on lithiation/delithiation and the associated mechanical stress, binder systems which can be processed aqueously, for example sodium carboxymethylcellulose (Na-CMC), polyvinyl alcohols, acrylates or mixtures of Na-CMC with styrene-butadiene rubbers, have been described as an alternative. Standard binder systems frequently display a high capacity loss over charging and discharging cycles, in particular at high loadings per unit area. In particular, a high irreversible capacity loss occurs during charging and discharging cycles. As causes of this, electrolyte decomposition and a resulting large rise in the internal resistance have frequently been discussed (Zhao et al., Journal of Power Sources (2014) 263, 288-295).
In anodes for lithium ion batteries in which the electrode active material is based on silicon as material having the highest known storage capacity for lithium ions, the silicon experiences an extreme volume change of about 300% during charging or discharging. This volume change results in a high mechanical stress on the total electrode structure and this leads to electronic decontacting of the active material and thus to destruction of the electrode with a capacity loss. Furthermore, the surface of the silicon anode material used reacts with constituents of the electrolyte with a continuous, irreversible lithium loss and (subsequent) formation of passivating protective layers (solid electrolyte interface; SEI).
To solve these problems which are known specifically for Si-based anodes, various approaches for the electrochemical stabilization of Si-based electrode active materials have been pursued in recent years (A. J. Appleby et al., J. Power Sources 2007, 163, 1003-1039).
With a view to applications in electromobility, the requirement for high reversibility at high cycle numbers (>100 cycles) and high loadings (>2 mAh/cm2) is still the greatest challenge.
Apart from the issue of decontacting due to binder weakness, electrolyte decomposition, in particular, plays an important role at high cycle numbers. Over the course of numerous cycles, this leads to a decrease in the capacity due to increasing resistances through to what is referred to as dry running of the cell.
Apart from specific electrolytes, the binder also plays an important role here and has to be appropriately adjusted in order to allow, in conjunction with further improvements such as suitable additives and improved electrolytes, use of silicon anodes in lithium ion batteries for, in particular, automobile applications.
PVDF as standard binder, as is employed in classical graphite anodes, is unsatisfactory in the case of silicon-containing anodes.
Zhao et al. (Journal of Power Sources (2014) 263, 288-295) describe the use of polymeric vinylene carbonate (polyVC) as binder for graphite anodes. PolyVC was already identified in previous work as the primary SEI component in graphite anodes (J. Electrochem. Soc. 156 (2009) A103-A113) In studies by Zhao et al. it was able to be shown that polyVC aids SEI formation and reduces electrolyte decomposition. Reduced electrolyte decomposition leads to a thinner SEI and thus to a slower increase in the internal resistance during cycling. In this way, a higher capacitive retention at high cycle numbers is achieved.
The use of polyVC as binder as in the abovementioned example for graphite anodes cannot be applied to silicon anodes. For reasons which have not been elucidated, polyVC in combination with silicon as anode active material leads to a rapid breakdown of the cycling stability.
The copolymerization of VC with acrylic acid and methacrylic acid has been described by Baskova et al. (Vysokomolekulyarnye Soedineniya, Seriya B: Kratkie Sooshcheniya (1968), 10(3), 220-2), and the copolymerization of VC with (meth)acrylic esters may be found, for example, in Smets et al (Jour. Pol. Sci., Vol 27, pp 275-283 (1958)).
Copolymers of vinylene carbonate and (meth)acrylic esters have hitherto been used in lithium ion batteries only as electrolyte constituent which can be polymerized in-situ in the cell (US2004/0126668 A1), but not as binder for the anode material.